It is obligatory to provide general or special electrical installations with protective devices to avoid the deleterious effects of overload for which these installations are not designed. These common protective devices are known as fuses, overvoltage protection tubes, lightning protection tubes and other similar names. Their purpose is to prevent transmitting an overload dangerous for a given type of installation.
The application of this type of protective device is of particular importance in the case of telephone installations, circuits and exchanges. Exchanges and the telephone lines are extremely vulnerable to lightning as well as to electrical stresses caused by induced overvoltages or to overloads caused by accidental contact between a power line and telephone line. Telephone exchanges have been completely destroyed after being struck by lightning or by overload due to the causes mentioned above.
In order to meet the requirement for protecting telephone circuits and exchanges, it has become necessary to connect a protective device, of the type commonly known as a lightning-protector, between each line and ground. The required charactersitics of this protective device are to cause no loss under normal operating conditions of the line, i.e. to present infinite resistance to current flow, and on the other hand to withstand and conduct to ground any transient overload. The lightning arrestor presents therefore a resistance which is always less than that of the circuit to be protected while maintaining a high current-carrying capacity above a predetermine voltage level.
Below a particular voltage designated as the striking voltage, a discharge tube presents infinite resistance, whereas for voltages across its terminals greater than the striking voltage the tube discharges to provide a low resistance. The tube is able to withstand high voltage overload, providing it has sufficient structure, by conducting these overloads to ground. The striking voltage value is easily predetermined by adjusting the dimensions of the tube discharge space. The current-carrying capacity is determined by the tube structure.
The manufacturer can therefore predetermined the various operating characteristics of the tube, which is a favorable factor for the use of a lightning-protector discharge tube.
Requirements are very stringent for the safety and reliability of telephone lines and circuits. In particular, one requirement for the parallel-connected protective device is that the latter forms a short-circuit whenever it becomes defective. If this requirement is not satisfied, nothing indicates failure of the protective device and the line would be destroyed by the first occurring overload. This can be avoided only by requiring the component to indicate its own failure. In this case, since the line then no longer operates, it becomes necessary to correct its defective protection by changing the faulty component in order to restore the line to normal operation. It is for this reason that the component must present a dead short-circuit when it can no longer perform its function, whatever the cause of failure. Grounding of the line makes it necessary to change the defective protection device.
In order to fulfill these conditions, this invention proposes a discharge tube characterized by the fact that is structurally possesses the means for automatically producing a short-circuit as soon as it is no longer able to operate in the normal discharge mode for any reason whatsoever.